Cerro Galán is a caldera in the Catamarca Province of Argentina. It is one of the largest exposed calderas in the world and forms part of the Central Volcanic Zone of the Andes, one of the three volcanic belts found in South America. One of several major caldera systems in the Central Volcanic Zone, the mountain is grouped into the Altiplano–Puna volcanic complex. Volcanic activity at Galán is the indirect consequence of the subduction of the Nazca Plate beneath the South America Plate, and involves the infiltration of melts into the crust and the formation of secondary magmas which after storage in the crust give rise to the dacitic to rhyodacitic rocks erupted by the volcano. Galán was active between 5.6 and 4.51 million years ago, when it generated a number of ignimbrites known as the Toconquis group which crop out mainly west of the caldera. The largest eruption of Galán was 2.08 ± 0.02 million years ago and was the source of the Galán ignimbrite, which covered the surroundings of the caldera with volcanic material. The volume of this ignimbrite has been estimated to be about ; after this eruption much smaller ignimbrite eruptions took place and presently two hot springs are active in the caldera. Geography and geomorphology The Galán caldera lies in the northwestern Catamarca Province of Argentina and was discovered in 1975 in a remote region of the Andes, using satellite images. The town of Antofagasta de la Sierra lies west-southwest of the Galán caldera, Tacuil is almost due northeast from the caldera and El Penon southwest of the volcano. Galán is part of the Central Volcanic Zone of the Andes, which lies on the western margin of South America, where the Nazca Plate subducts beneath the South America Plate. There are about 50 volcanoes with recent activity in the Central Volcanic Zone, and additional volcanoes exist in the Northern Volcanic Zone and the Southern Volcanic Zone, two other volcanic belts north and south. The volcanic arc runs along the borders between Bolivia and Argentina with Chile, and behind the volcanic arc lies a chain of silicic volcanoes, of which Galán is a southern member. The whole region has been subject to substantial ignimbrite-forming volcanism with many eruptions producing volumes of rock larger than , although the actual vents often are only visible from space imagery. Many vents cluster in an area known as the Altiplano-Puna